By Dave Scherping
A few weeks ago, Martin Gaskel invited me over to measure the reflectivity of several secondary mirrors. I brought along the new Galaxy secondary from the club scope and the secondary from my 10″ scope. My secondary has standard coatings (I forgot to include it when I had my primary mirror enhanced coated) and the club scope’s secondary has enhanced coatings. We also planned to evaluate Martin’s secondary from the famous “Tel-Poke”.
Martin had borrowed a photometer from UNL to make the measurements. The test set-up was rather simple, a light source was set-up at one end of the room and the photometer was placed at the other end. The intent was to compare measurements of the amount of light the photometer gathered with and without the mirror in the light path. The photometer gives a numerical readout in voltage and has a shutter that opens and closes the front end of the unit. A lens was placed over the front of the photometer to focus the light. A red LED was placed about 25 feet away and was surrounded by black construction paper to reduce glare.
The first step was to take a measurement without the mirror in place. To do this, we placed the photometer at the distance where the LED comes to focus and then took two measurements, one with the shutter open and one with it closed. Subtracting one from the other gives the amount of light from the LED alone. (The measurement with the shutter closed is a measurement of noise and stray light). Note that all measurements were taken with all other lights in the room turned off.
We then repeated the above steps with the mirror placed in the light path, being careful to keep the distance of the light path consistent. The values recorded with the mirror in place were, of course, lower than those obtained without the mirror. The ratio of the two is the reflectivity of the mirror. The tests were conducted twice for each mirror. The Tel-Poke mirror was measured before and after cleaning, and showed a significant decrease in reflectivity when the mirror was dirty. I think when you see the results below, you’ll get out the distilled water, dish soap, & cotton!
A few nights later, Jason Stahl took his secondary to Martin’s and measured it in the same manner. Jason’s mirror was coated at QSP with their Endurobrite (96%) coating. The following results were obtained for the mirrors tested:
Dave’s Secondary: 84.2% 84.9% Club Scope Secondary: 94.4% 93.9% Tel-Poke (dirty): 72.2% Tel-Poke (cleaned): 83.0% Jason’s Secondary: (3.1") 95.9% (1.8") 95.3%